1. Field of the Invention
This invention relates to laser systems and more specifically concerns a laser power supply of reduced starting power.
2. Description of Related Art
Power supplies for high pressure gas lasers often employ a radio frequency or RF pumping signal generating system having a frequency established at the desired laser excitation or pumping frequency. Examples of such radio frequency excitation systems are shown in the U.S. Patent to Laakmann, U.S. Pat. No. 4,169,251, and the U.S. Patent to Angle, et al, U.S. Pat. No. 4,451,766.
The gas laser, like other gas discharge devices, requires a much higher starting voltage for ignition of the cold laser than is required for the normal operation or running. Prior art power sources have relied on fixed element impedance matching circuits that have been adjusted to provide a compromise between the impedance match best for applying a high starting voltage to the cold gas discharge device and the impedance match required for efficient, steady operation. Such compromise circuits can be optimized for neither starting nor efficient running. As a proposed solution to this limitation, and in an attempt to provide an RF power supply that provides a high voltage for starting and also an impedance match between the power supply and the gaseous medium of the laser for high efficiency of energy transfer during steady state operation, the above identified patent to Angle, et al, provides a variable impedance matching circuit. In this arrangement, change in impedance of the laser cavity is sensed, and the sensed change used to vary impedance of the matching circuit. This circuit may require continuous adjustment of the radio frequency source. Further, in some applications, variations in impedance of the gas discharge device, the laser, may produce significant unwanted variations in the radio frequency. For example, RF excited gas lasers usually operate at 40.68 MHz owing to FCC requirements on RF emission. Variation of the RF excitation to change laser output power may produce up to 0.5 MHz frequency change, resulting in violation of FCC emission regulations. Further, in certain military systems not subject to FCC regulations, laser drive frequency may be restricted to prevent detrimental interference with other system components.
Still another limitation of a system such as that of the Angle, et al patent, is the fact that the variable component impedance matching circuit necessarily operates on the high power output of the power supply. As power levels increase, it becomes more and more difficult to obtain circuit elements which can be appropriately varied for providing the desired variable impedance matching. Required high power components are less stable and more costly. Thus, while the system of the Angle, et al patent may be applicable for RF amplifiers in the power range of about 50 watts, it is much more difficult to apply the system to higher power amplifiers, those of up to about 300 watts and higher, for example.
Although improved efficiency is obtained in the prior patent of Angle, et al, the system still requires high power RF supplies, capable of handling the very high power required for igniting the laser.
Accordingly, it is an object of the present invention to reduce required starting power of a laser and to avoid or minimize above-mentioned problems.